1. Field of the Invention
The present invention relates to a telescopic rod with a plurality of tubular rod sections which are mounted so as to be slidable within each other, and with a multistage piston-cylinder unit arranged within the telescopic rod. The multistage piston-cylinder unit is connected, on the one hand, to the outermost rod section of the telescopic rod and, on the other hand, to the last inner rod section of the telescopic rod.
2. Description of the Related Art
Telescopic rods of the above-described type are used, for example, as telescopic jibs in a vehicle crane or rail crane, as a jib of a truck loading crane or as a telescope arm of a work platform. The telescopic rod has a plurality of tubular rod sections which are slidable into one another. The cross-section of these tubular rod sections may be round, oval or polygonal. In the latter case, the rod sections are box-shaped sectional members. The multistage piston-cylinder unit used for extending the telescopic rod is mounted within the rod and the number of stages may correspond to the number of rod sections of the telescopic rod. However, this does not necessarily have to be the case. For retracting the telescopic rod, usually a separate return device is provided. This return device may be composed of a rope or a chain which is arranged within the telescopic rod and is connected, on the one hand, to the last extendable rod section and, on the other hand, to a winding device.
When the telescopic rod or the piston-cylinder unit are extended, the resistance against collapsing stress applied to the extended piston rod is decreased in proportion to the extent by which the telescopic rod has been extended, i.e., in dependence on the length of the extended portion of the piston rod or the effective length of each rod section. Accordingly, an increasing length of the telescopic rod in the extended state requires an increase in the resistance against collapsing stress. Since cranes with great lifting heights are required in actual use, each rod section of the extendable telescopic rod must have a very great length and, consequently, a piston-cylinder unit is required which has a very long piston rod, so that one of the long rod sections can be extended and retracted relative to another of the other rod sections over the necessary distance. When the extended portion of the piston rod has a very long length, usually a piston rod with a great diameter is provided which has the necessary resistance to buckling. However, when the diameter of the piston rod is increased, a cylinder must be provided whose dimensions are also greater, so that the cylinder has a greater weight and the manufacturing costs are increased.
Accordingly, it has been attempted to develop means for preventing buckling of an extended piston-cylinder unit in a telescopic rod. Means of this type are described and illustrated in DE-OS 20 18 926. In that case, a carriage is provided which is displaceably mounted on the piston rod of the piston-cylinder unit. The carriage has radially outwardly directed rigid arms which rest through rollers or slide blocks against the inner side of the respectively following rod section. The carriage is provided with drive means which are guided through transmissions in a such a way that, in the case of a given extended distance of two rod sections, the carriage guided on the piston rod travels only half this given extended distance. This is effected partially by means of cable pull devices, partially by means of racks and pinions or by means of drive members acting against spring pull devices. All of these measures are structurally complicated and, therefore, not useful.